The present invention relates to a humidity meter, in particular, relates to a humidity meter which has a humidity sensor with electrical impedance depending upon humidity, and an electronic circuit to process measured humidity signal.
A humidity meter of electrical impedance type is shown in U.S. Pat. No. 5,065,525, and U.S. patent application Ser. No. 07/986,688, assigned to the same assignee as the present application (EP patent application 92 310877.3).
A humidity meter of electrical impedance type must have two electronic circuits. A first circuit is to function to convert electrical impedance which depends upon humidity to electrical DC (direct current) potential which is the indication of measured humidity. A second circuit is to process said DC potential for various purposes. Some examples of the process are analog-to-digital conversion of DC potential of measured humidity in analog type to digital type signal, visible indication of converted digital signal, and/or interface for supplying the digital signal to an external circuit.
A first circuit is small and consumes less power, and therefore, it is assembled in a sensor assembly together with a sensor element itself. On the other hand, a second circuit is large, consumes much power thereby dissipates much heat.
When we try to mount a sensor element of above type and related electrical circuits in a single housing, the sensor element itself must be disposed in open air to measure humidity in open air, and must be isolated or separated completely in the housing from the electrical circuit which dissipates much heat so that the measured humidity is not affected by heat generated in the electrical circuit. If above conditions are not satisfied, accurate measurement of humidity would be impossible.
Simultaneously, the structure of humidity meter which satisfies above conditions must be simple.
A humidity meter which satisfies above conditions has not been known, although a humidity sensor element of impedance type, and electrical circuits for processing an output of a sensor element have been known.